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    <title>Document</title>
</head>
<body>
    
    <button id="add">+</button>
    <input type="text" value="0"> 
    <button id="div">-</button>
    <script>
        let input = document.querySelector('input');
        let add = document.querySelector('#add');
        let div = document.querySelector('#div');
        add.onclick = function() {
            input.value = parseInt(input.value)+1;
        }
        div.onclick = function() {
            input.value = parseInt(input.value)-1;
        }
    </script>
</body>
</html>